(1) Field of the Invention
The present invention is directed to stator blades and rotor blades on an underwater vehicle. In particular, the present invention is directed to an apparatus to reduce stator blade rate tonal noise through altering the mean and instantaneous characteristics of the rotor blade inflow.
(2) Description of the Prior Art
Conventional stator blades on the hulls of underwater vehicles add swirl to the propeller inflow that increases the efficiency of the propeller through the cancellation of the swirl generated by the propeller. However, stator blades and other upstream appendages also produce discrete wakes due to the velocity deficit caused by viscosity of the fluid moving over the stator blade surface. When a propeller blade meets these velocity deficits during the rotation of the propeller, sudden variations of force are produced. These regular unsteady force disturbances create a recognizable noise, the blade rate signature. This noise can be used to detect and identify the vehicle. Experiments using “swirl inducing stator upstream of propeller propulsors” have highlighted the effect of sharp stator wakes on propulsor radiated noise. Anytime the inflow to the rotor is circumferentially unsteady, caused by the wakes of upstream appendages, control fins, guide vanes, or stators, for instance, the loading on the rotor blades is unsteady and periodic with the blade rate. In the case of swirl inducing stator upstream of propeller propulsors the sharp wake deficits behind the stators result in unsteady loading and distinguishable peaks in the noise spectra at harmonics of the blade rate, with the frequency of the peaks in the spectra dependent on the number of stator and rotor blades. This radiated noise signature can be used to classify vehicles using sonar.
Swimming and flying animals use flapping wings or fins to produce thrust and maneuvering forces. They do this through the creation of an alternating vortex wake, similar to the drag wake seen behind cylinders in a flow but with opposite sign. The discrete vortices form a thrust jet through the center of the vortex street which accelerates fluid away from the fin, accelerating the animal forward. This same effect can be used to fill a stator or guide vane wake, but rather than create a vortex wake powerful enough to produce net thrust as is the case with animal fins, the stators can be flapped just enough to overcome its own drag and fill its wake deficit.
Currently, there is a need to reduce or eliminate the radiated noise signature of rotors through the use of a simple flapping motion of the trailing edge of a stator blade in order to fill its mean wake deficit to reduce unsteady loading on the rotor blades. Interaction between the rotor blades and the discrete vortices in the flapping stator wake may increase unsteady loading on the rotor without careful timing. Through active control of the stator oscillations the timing of rotor blades passing through the wake can be chosen to minimize the periodic loads on the rotors. This reduction in the unsteady loading will diminish blade rate harmonic tones.